Implications of matrix diffusion on 1,4-dioxane persistence at contaminated groundwater sites

Authors: D. Adamson, P. de Blanc, S. Farhat, C. Newell
Published: March 2016 in Science of the Total Environment volume 562 (2016) pages 98-107.

Abstract

Management of groundwater sites impacted by 1,4-dioxane can be challenging due to its migration potential and
perceived recalcitrance. This study examined the extent to which 1,4-dioxane's persistence was subject to
diffusion of mass into and out of lower-permeability zones relative to co-released chlorinated solvents. Two
different release scenarios were evaluated within a two-layer aquifer system using an analytical modeling approach.
The first scenario simulated a 1,4-dioxane and 1,1,1-TCA source zone where spent solvent was released.
The period when 1,4-dioxane was actively loading the low-permeability layer within the source zone was estimated
to be b3 years due to its high effective solubility. While this was approximately an order-of-magnitude
shorter than the loading period for 1,1,1-TCA, the mass of 1,4-dioxane stored within the low-permeability
zone at the end of the simulation period (26 kg) was larger than that predicted for 1,1,1-TCA (17 kg). Even
80 years after release, the aqueous 1,4-dioxane concentration was still several orders-of-magnitude higher
than potentially-applicable criteria. Within the downgradient plume, diffusion contributed to higher concentrations
and enhanced penetration of 1,4-dioxane into the low-permeability zones relative to 1,1,1-TCA. In the second
scenario, elevated 1,4-dioxane concentrations were predicted at a site impacted by migration of a weak
source from an upgradient site. Plume cutoff was beneficial because it could be implemented in time to prevent
further loading of the low-permeability zone at the downgradient site. Overall, this study documented that 1,4-
dioxanewithin transmissive portions of the source zone is quickly depleted due to characteristics that favor both
diffusion-based storage and groundwater transport, leaving little mass to treat using conventional means.